1. Field of the Invention
The invention relates to a method of fabrication of preforms for manufacturing of optical fibers based on surface plasma wave chemical vapor deposition (CVD) of doped or undoped silica layers on the outside surface of a dielectric starting body of large cross section. Equipment to carry out this process is also described.
2. Information Disclosure Statement
Numerous methods and devices have been proposed to fabricate preforms for the manufacture of optical fibers by plasma CVD. Plasma assisted chemical vapor deposition is superior to classical CVD as far as the efficiency of gas usage is concerned. It also allows higher deposition rates, more reasonable pressures and higher fluorine dopant concentrations in the quartz glass matrix than the conventional alternatives, a feature of practical interest for radiation resistant fibers.
Plasma CVD for preform manufacturing by deposition on the inside of a tube is described in U.S. Pat. No. 4,714,589. A resonator generating a plasma in the inside of the tube is moved mechanically relative to the tube in an axial direction. This plasma dissociates the precursor gas in the tube and leads to the deposition of successive layers of doped or undoped quartz glass on the inner walls with each pulse of the plasma. The number of layers influences the optical quality of preform as the desired parabola-like and smooth refractive index profile needs to be approximated by successive steps. Moving the heavy resonator back and forth mechanically cannot be done practically with speeds exceeding 5-10 m/min, therefore the number of layers achievable by this method is limited. The mechanical construction of the machinery is complicated, and the operating reliability of the process compromised.
Another apparatus and method for plasma CVD of doped or undoped quartz layers on the inside of a starting tube is described in U.S. Pat. No. 4,944,244. This method uses a microwave plasma source whose power is modulated between a minimum and a maximum value leading to the back-and-forth movement of a plasma column inside the tube. As precursor gases are conducted through the tube they dissociate at the plasma front and a layer of silica is thus deposited with each stroke. In such a method no moving parts are required to move the plasma. Problems exist however with the homogeneity of the deposition over the whole length and the U.S. Pat. No. 4,944,244 patent describes an elaborate feed back control mechanism that is intended to overcome these problems. While the patent does not offer any clues as to which waves are excited, it can be assumed that it operates in a multimode regime. It may well be that for the inside deposition with the described feedback mechanism this does not matter indeed.
While this arrangement is clearly superior to the moving resonator arrangement described earlier as no moving mechanical parts are required in the machinery, thus enhancing its reliability and the number of layers that can be deposited in a given period of time, it suffers from the drawback that an elaborate feed back control mechanism is required to assure uniform deposition over the preform length.
All the methods described above deposit on inside walls of a tube and are therefore limited in deposition speeds by the amount of fresh gas that can be brought to the deposition site through the limited cross section of the tube. High concentrations of certain dopants (for example fluorine) are difficult to achieve with the inside deposition methods, due to the high tensile forces generated in the quartz glass body. These forces can lead to fracturing and destruction of the glass body. Furthermore for the manufacturing of fibers with a large core to clad ratio it is much preferable to deposit on the outside of a commercially available pure quartz starting rod.